1. Field of the Invention
This application claims priority from Japanese Patent Application No. 2011-148201 (filed on 4 Jul. 2011), the contents of which are incorporated entirely herein by reference.
The present invention relates to a piezoelectric ceramic having an alkali-containing niobate-based perovskite structure but free of lead and the like, and a multi-layer piezoelectric ceramic element to be obtained by laminating alternately a plurality of piezoelectric ceramic layers composed of the piezoelectric ceramic and electrodes.
2. Description of the Related Art
The principle of conversion of electrical energy to mechanical energy and mechanical energy to electrical energy (piezoelectric effect) of a piezoelectric ceramic is applied to many electronic devices. Such an electronic device utilizing the piezoelectric effect is referred to herein as a “piezoelectric device,” and an electronic element with a piezoelectric ceramic to be usable in such a piezoelectric device is referred to as a “piezoelectric ceramic element.”
For a piezoelectric ceramic element, a piezoelectric ceramic containing lead and composed of 2 components, such as PbTiO3-PbZrO3 (hereinafter referred to as “PZT”) or a piezoelectric ceramic yielded by adding a third component, such as Pb(Mg1/3Nb2/3)O3 or Pb(Zn1/3Nb2/3)O3, to the PZT has been heretofore used. Piezoelectric ceramics containing PZT as a main component exhibit a good piezoelectric property, and are employed currently in many piezoelectric ceramic elements already in practical use. However, since piezoelectric ceramics utilizing PZT as a main component contain Pb, there has been a problem of high environmental load due to volatilization of PbO, etc. in the production process.
Consequently, lead-free or low-lead piezoelectric ceramics have been demanded. Lead-free piezoelectric ceramics have been studied recently energetically. For example, Nature, 432 (4), 2004, pp. 84-87 and Applied Physics Letters 85 (18), 2004, pp. 4121-4123 have reported that a piezoelectric ceramic having an alkali-containing niobate-based perovskite structure exhibits a piezoelectric effect equivalent to PZT. The piezoelectric ceramic contains Li, Na, K, Nb, Ta, Sb and O as main components and is expressed by a general formula {Lix[Na1-yKy]1-x}a{Nb1-z-wTazSbw}bO3 (wherein x, y, z, w, a and b mean molar fractions, and 0≦x≦0.2, 0≦y≦1, 0≦z≦0.4, 0≦w≦0.2, 0.95≦a, b≦1.05). It has been commonly known that a piezoelectric ceramics expressed by the compositional formula has good piezoelectric properties, such as piezoelectric constant, and electromechanical coupling factor, (see Japanese Patent Laid-Open No. 2002-068835, Japanese Patent Laid-Open No. 2003-342069, and Japanese Patent Laid-Open No. 2004-300012).
While, a multi-layer piezoelectric ceramic element utilizing a piezoelectric ceramic expressed by the compositional formula has been under development. For example, Japanese Patent Laid-Open No. 2008-207999 discloses a multi-layer piezoelectric ceramic element, for which low temperature firing is made possible by mixing Li2CO3, LiBO2, or Li2B4O7 as a sintering agent. Further, International Publication No. WO 2008/152851 discloses a piezoelectric ceramic composition expressed by a general formula {(1−x)(K1-a-bNaaLib)(Nb1-cTac)O3-xM2ZrO3} (provided that M2 is at least one of Ca, Ba, and Sr; x, a, b, c are respectively 0.005≦x≦0.1, 0≦a≦0.9, 0≦b≦0.1, 0≦a+b≦0.9, and 0≦c≦0.3), wherein Mn is contained in a range of 2 to 15 mol with respect to 100 mol of the main component, and Zr is contained in a range of 0.1 to 5.0 mol with respect to 100 mol of the main component, as well as a multi-layer piezoelectric ceramic element having a piezoelectric ceramic layer formed by the piezoelectric ceramic composition.
Further, Japanese Patent Laid-Open No. 2010-180121 discloses a multi-layer piezoelectric ceramic element comprising a piezoelectric ceramic layer having a core phase and a shell phase of compositions different from each other, wherein a piezoelectric ceramic is expressed by the general formula {Lix[Na1-yKy]1-x}a{Nb1-z-wTazSbw}O3 (provided: 0≦x≦0.2, 0≦y≦1, 0≦z≦0.4, 0≦w≦0.2, x+y+z>0, and 0.95≦z≦1).
Further, Japanese Patent Laid-Open No. 2010-52999 discloses a piezoelectric ceramic expressed by the general formula (1−a)[K1-xNax]1-yLiy[Nb1-z-wTazSbw]O3+aK3Nb3O6Si2O7 (provided: 0≦x≦1, 0≦y<1, 0≦z<1, 0≦w<1, and 0.003≦a≦0.1) as a material for a multi-layer piezoelectric ceramic element. In a piezoelectric ceramic disclosed in the same application, the average grain size of polycrystals constituting the ceramic can be suppressed to 0.8 to 3 μm. Therefore, the multi-layer piezoelectric ceramic element produced from the piezoelectric ceramic has a larger amount of crystal grains between the layers. As the result, the multi-layer piezoelectric ceramic element has a larger number of grain boundaries between the layers. Since in a piezoelectric ceramic sintered sufficiently compact, the insulation property at a grain boundary is generally superior to the intragranular insulation property, by increasing the number of grain boundaries between the layers, the resistance property can be improved. Further, the piezoelectric ceramic disclosed in the same application has good piezoelectric properties, such as an electromechanical coupling factor kp higher than 38%. The electromechanical coupling factor is an index of efficiency of conversion of electrical energy to mechanical energy.